Is returning a value != '0' or '1' as jboolean from a JNI function legal?

[Volker Simonis]

On Mon, Aug 20, 2018 at 6:09 PM, Krystal Mok <rednaxelafx at gmail.com> wrote:
> Hi guys,
>
> Haha this is fun. I actually hit this issue the hard way and had to tweak a
> bit of my code to accommodate that: I had to return a jint from a function
> that I wanted to return a jbool at first:
> http://hg.openjdk.java.net/hsx/hsx25/hotspot/diff/8f37087fc13f/src/share/vm/c1/c1_Runtime1.cpp
>

Interesting, but I think that mis-behavior should have been fixed
meanwhile by the infamous "Better byte behavior" change [1] (which was
a security fix, so you won't find too much information about it).

So maybe you can remove your workaround and comment now :)

[1] http://hg.openjdk.java.net/jdk/jdk/rev/291ee208fb72

> - Kris
>
> On Mon, Aug 20, 2018 at 8:38 AM Volker Simonis <volker.simonis at gmail.com>
> wrote:
>>
>> On Mon, Aug 20, 2018 at 4:55 PM, Aleksey Shipilev <shade at redhat.com>
>> wrote:
>> > On 08/20/2018 12:22 PM, Volker Simonis wrote:
>> >> So to summarize, my current view on this topic is:
>> >>  - JNI functions returning a jboolean are only allowed to return
>> >> JNI_TRUE/JNI_FALSE (or 1/0) according to the current JNI spcification.
>> >
>> > Now *I* am having trouble seeing where exactly the JNI spec says the
>> > domain of jboolean is
>> > (JNI_FALSE, JNI_TRUE). In "Primitive Types" [1] it says "The following
>> > definition is provided for
>> > convenience: JNI_FALSE, JNI_TRUE", but that does not restrict the
>> > domain, because those are
>> > "convenience" defines. And "Description" in the table says jboolean is
>> > "unsigned 8 bits", which
>> > seems to invite interpretation that all 8 bits are usable.
>> >
>> > John says [2]:
>> >
>> > "The JNI documents specify that, at least for returning values from
>> > native methods, a Java boolean
>> > (T_BOOLEAN) value is converted to the value-set 0..1 by first truncating
>> > to a byte (0..255 or maybe
>> > -128..127) and then testing against zero."
>> >
>> > ...which is what I am looking for, but I cannot find the "JNI document"
>> > that actually says that. I
>> > can see the idea of that in JVMS [3], but that seems to only apply to
>> > on-heap booleans, does that
>> > also extend to jboolean's? Maybe John can point out the JNI document
>> > where it is said explicitly?
>> >
>>
>> Yes, you're right - there's no exact documentation for neither of the
>> two possible interpretations. A colleague just pointed me to the
>> definition of invokestatic in the JVMS [4] which has the following
>> sentence:
>>
>> "If the native method returns a value, the return value of the
>> platform-dependent code is converted in an implementation-dependent
>> way to the return type of the native method and pushed onto the
>> operand stack."
>>
>> But then again, it has this unfortunate "implementation-dependent"
>> which can be interpreted either way :(
>>
>> [4]
>> https://docs.oracle.com/javase/specs/jvms/se10/html/jvms-6.html#jvms-6.5.invokestatic
>>
>> >
>> >>  - to code in Java_java_io_Console_echo() should be fixed (as
>> >> confirmed by Sherman later in this thread)
>> >
>> > Yes, that's a bug waiting to happen anyway.
>> >
>> >
>> >>  - normalization of native, off-heap 8-bit values to Java booleans as
>> >> currently implemented in the HotSpot (and fixed by JDK-8161720) is (1)
>> >> only for convenience to simply access to off-heap data in Unsafe, (2)
>> >> to implement better Java/Native integration in projects like Panama
>> >> and (3) to fix legacy JNI code which was developed under the
>> >> assumption that the advice in the "JNI Programmer's Guide &
>> >> Specification" book is specification relevant.
>> >
>> > Yes, the intent seems to be what you describe. But see above about the
>> > spec.
>> >
>> >
>> > -Aleksey
>> >
>> > [1]
>> > https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/types.html#primitive_types
>> > [2]
>> > http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2016-August/024263.html
>> > [3]
>> > https://docs.oracle.com/javase/specs/jvms/se10/html/jvms-2.html#jvms-2.3.4
>> >